Modern internal combustion engines for operating motor vehicles feature lambda probes which measure the air ratio (combustion air ratio) in the exhaust gas of the internal combustion engine in order to be able to take account of the air ratio in the open-loop or closed-loop control of the internal combustion engine.
Linear lambda probes are used for this purpose for example, which are raised by an electrical heater to the necessary operating temperature of typically 650° C.-850° C. The desired effective heat output of the electrical heater is mostly set in such cases by a pulse-width-modulated (PWM) control method in which the electrical heater is activated in accordance with a predefined pulse duty ratio alternately with two different voltages of for example 0V (ground) and 14V (battery voltage).
Under specific peripheral conditions undesired electrical connections can be produced between the electrical terminal contacts of the electrical heater and the output contacts of the lambda probe. For example under some circumstances moisture can penetrate into the plug-in connector between the lambda probe and the engine controller. In addition manufacturing faults can also cause the undesired electrical connections between the electrical terminal contacts of the electrical heater and the output contacts of the lambda probe.
Depending on the electrical transfer resistance of the undesired electrical connections such a fault can produce more or less strong effects on the output signal of the lambda probe. Thus the useful signal of the lambda probe is overlaid in the event of such a fault by a noise signal emanating from the pulse width modulated control signal of the electrical heater of the lambda probe, so that the noise signal has a frequency corresponding to the clock frequency of the pulse-width-modulated control signal of the lambda probe.
The fault described here of the lambda probe because of the heater input can significantly adversely affect the function of the lambda controller, since the lambda controller receives an incorrect air ratio as its input signal. Depending on the intensity of the fault the result in such cases can be negative influences on the driving behavior (e.g. jerking) or a deterioration in the exhaust gas values.
The diagnosis of a fault-related heater input for a heated lambda probe is therefore known from the prior art. The known diagnosis methods for detecting a heater input are based on an evaluation of the output signal of the lambda probe, with the evaluation being synchronized with the clock frequency of the pulse-width-modulated control signal of the lambda probe heater. The call-up rate of the diagnosis needed for a reliable fault detection is determined in such cases from the clock frequency of the pulse-width modulated control signal of the lambda probe heater, which typically lies between 10 Hz and 100 Hz.
The disadvantage of the known diagnosis method is thus the fact that, above all at high clock frequencies of the pulse-width modulated control signal of the lambda probe heater, a large computing run time is needed.
A dynamic diagnosis of an exhaust gas probe is known from DE 10 2005 032 456 A1 to detect an ageing-related deterioration of the dynamic behavior of the exhaust gas probe, in which it is also disclosed that the lambda controller intervention can be evaluated. However the detection of a heater input is not known from this publication.
Furthermore DE 100 56 320 A1 and EP 0 624 721 A1 disclose diagnostic methods in conjunction with lambda probes which however likewise do not make it possible to detect a heater input.
A conventional method for detecting a heater input is likewise known from DE 198 38 334 A1. In this case only the output signal of the lambda probe is evaluated which is associated with the known problems.